1. Field of the Invention
The present invention relates generally to a wafer cleaning technique in semiconductor fabrication and, more particularly, to a method of cleaning a silicon nitride layer so as to effectively remove impurities such as polymer and particle from the silicon nitride layer.
2. Description of the Related Art
As is well known in the art, a silicon nitride layer has a denser and tighter structure in comparison with a silicon oxide layer, so the silicon nitride layer has excellent ability to prevent infiltration of moisture or alkali ions, and thus is typically used as a passivation layer. Furthermore, the silicon nitride layer is not oxidized and prevents oxidation of underlying silicon layer, so the silicon nitride layer is often used as a masking layer for selective oxidation. The silicon nitride layer is also used as an ion implant mask for suppressing diffusion of oxygen or other elements during the ion implantation process.
Additionally, the silicon nitride layer has positive zeta potential and strongly attracts negatively-charged impurities such as polymer and particle, so such impurities can be hardly removed from the silicon nitride layer.
FIG. 1 is a diagrammatic view illustrating zeta potential. As shown in FIG. 1, a solid 10 has electric charges in general at its surface 11 that adjoins an aqueous solution 12. Such charges near the solid surface 11 attract reversely charged ions existing in the aqueous solution 12, so an electrical double layer 13 is formed near the solid surface 11.
Here, the potential at the solid surface 11 is referred to as the surface potential (Ψ0). Further, a degree of diffusion of reversely charged ions is represented by the Debye parameter (κ), and its inverse (κ−1) represents the thickness of the electrical double layer 13.
The zeta potential (ζ) means the potential at the outermost of the electrical double layer 13. In most cases, the zeta potential (ζ) is similar with the Stem potential (ΨS) that means the potential at the Stem plane.
FIG. 2 is a graph showing the zeta potentials of a silicon nitride layer, an HDP-CVD oxide layer, and a silicon substrate. As is appreciated from FIG. 2, the silicon nitride layer exhibits positive zeta potential regardless of pH.
Therefore, as discussed above, the silicon nitride layer has the force of strongly attracting negatively-charged impurities such as polymer and particle. So, although a cleaning process is performed, such impurities can be hardly removed from the silicon nitride layer.
Conventional techniques of removing impurities from the silicon nitride layer include a spin scrubber cleaning method, a chloric/ozone mix cleaning method using a mixed solution of hydrochloric acid (HCl) and ozone (O3), and a SC-1 (standard cleaning-1) method using a mixed solution of ammonium hydroxide, hydrogen peroxide, and water.
However, these techniques are separately employed in general and do not use electrochemical surface properties of the silicon nitride layer. So these conventional techniques may fail to provide effective cleaning for the silicon nitride layer.